Telecommunications systems, and particularly telephony systems, inherently include several sources of echo. Echo is created by the junction between the transmitting terminal and the telephone network, between the telephone network and the remote terminal, and at one or more places along the network. A standard requirement for such telecommunications systems is echo cancellation.
While echo cancellors vary greatly in complexity, design, and effectiveness, the basic idea behind the implementation of such echo cancellors is substantially the same. That is, at the source of the original signal (i.e. the transmitting terminal) an estimate of the echo is derived. The estimate is then subtracted from the actual echo being created by the system, resulting in the actual echo being canceled.
One major goal of all echo cancellors is to accurately estimate the actual system echo in as little time as possible. This process, known as training, should be as short as possible because during the training period, the echo cancellor is not functioning properly. Thus, the signal processing profession has put much effort into devising algorithms that can accomplish rapid training of echo cancellors.
A factor that complicates the training and the overall design of echo cancellors is that the echo originates from several different sources. The time it takes for the echo to travel from the location in the network at which it originates to the transmitting terminal must be accurately estimated. Since the echo is derived from several sources, echo arrives at the transmitting terminal at different times, and the amount of echo may be different for each of the several sources. Thus, the echo cancellor must train to cancel echo from different sources having different characteristics, all in a relatively small amount of time.
One prior art technique utilized to train an echo cancellor to cancel the multiple echoes discussed above is disclosed in the article “A DSP Based Long Distance Echo Cancellor Using Short Length Centered Adaptive Filters” to Marques et al. Marques discloses a set of echo cancellors that are used in combination with delay estimators. The delay estimator sweeps through various delays until it determines what is the delay that most accurately estimates the actual time that the echo takes to reach the transmitting terminal. The problem with such an approach is that it increases the time required for the echo cancellor to train. Specifically, for each source of echo, the entire set of possible delays must be scanned. This results in increased use of processing power, and extended training time.
In view of the above, there exists a need in the art for an improved technique of providing an echo cancellor that can rapidly train to cancel echoes from a plurality of sources in a telecommunications system. The training process should ideally require a relatively minimal amount of processing overhead.